Impurities are implanted into semiconductor devices for a variety of reasons, including introducing electrons and holes into the semiconductor substrate in order to locally change the conductive properties of the substrate. For example, silicon has four electrons in the outer ring. Phosphorus has five electrons in its outer ring, one more than silicon. Boron has three electrons in its outer ring, one fewer electron than silicon. Boron can be used to introduce holes into the substrate. Phosphorous can be used to introduce electrons into the substrate.
To enable implantation, the impurities are implanted as ions having one fewer electron than the neutral species. During the implantation process, the electron deficit can be used to determine how much impurity has been implanted. Specifically, it is not possible to accurately count the number of ions (or atoms) leaving the ion gun. Therefore, a predetermined portion of the ions is directed to an ion counter instead of the semiconductor wafer(s). The ion counter may be embodied has a disk faraday. When an ion strikes the disk faraday, an electron is pulled to the disk faraday in order to neutralize the ion. The number of electrons pulled to the disk faraday is counted using a current meter. It is presumed that the number of ions striking in the disk faraday is proportional to the number of ions striking and entering the semiconductor wafer.
The current (electrons per second) represents the rate at which impurities are introduced into the wafer. If the implanter detects that one area of the wafer is receiving impurities at a slower rate than other areas of the wafer, then the implanter spends more time implanting on the deficient area. In this manner, the implanter can work to achieve uniform total dosing across the surface of the wafer.
When the ions hit the semiconductor wafer, they may destroy a portion of a resist layer formed on the wafer. This process releases an outgas into the implant chamber, which would otherwise kept at a very low pressure. Electrons from the outgas can neutralize a portion of the ions, before the ions reach the disk faraday or the semiconductor wafer. Although the ions are neutralized by the resist outgas (rather than being neutralized at the disk faraday or within the semiconductor wafer), the neutral species is still implanted and still causes the desired change to the substrate. However, because the neutral species contains the correct number of electrons, there is not disk faraday current flow for neutralization. Therefore, the neutral species are not counted.
In order to count the impurities implanted as atoms, rather than ions, a pressure sensor is used. As the pressure increases from resist outgassing, it is presumed that a larger percentage of the impurities are introduced into the wafer as atoms rather than ions.
The following equation represents how pressure is taken into consideration to determine the number of ions implanted.IDISK=IDOSE·e−KP
In the above equation, IDISK is the current flowing to the disk faraday. This current is proportional to the number of ions implanted. IDISK is the rate at which impurities (ions+atoms) are implanted. P is the pressure as sensed by the ion gauge/pressure sensor within the device. K is a factor determined by the engineer and input into the implanter. K represents how a pressure change is presumed to effect ion neutralization.
Instead of, or in addition to, the K-factor shown above, a pressure compensation factor P-COMP can be used. The mathematical relationship between K and P-COMP is as follows:             P      -      COMP        =          100      ⁢              (                              ⅇ                          K              /              10000                                -          1                )              or      K    =                  ln        ⁡                  (                      1            +                                          P                -                COMP                            100                                )                    ⁢              (        10000        )            
Because K and P-COMP are interchangeable through simple math, the term “pressure compensation factor” is used hereinafter to represent both K and P-COMP with the understanding that the two parameters are interchangeable through the above mathematical relationships.
Conventional methods for determining the pressure compensation factor K (or P-COMP) have several disadvantages. 1) Wafers must be implanted, annealed, and measured. This takes time, incurs implanter downtime, and costs money in test wafers. 2) The conventional method uses wafer mean sheet resistance values to determine the pressure compensation factor. K (or P-COMP) does not take wafer non-uniformity into account. Using mean sheet resistance is especially disadvantageous since the purpose of the procedure is to minimize wafer non-uniformity. 3) The conventional method requires patterned resist dummy wafers and one bare silicon test wafers. This consumes photolithography tool time. 4) The conventional method does not test whether the pressure compensation factor is optimum for actual product wafer runs. This prevents the process engineer from easily verifying dose (number of ions plus number of atoms) uniformity without the use of parametric device data, which is a time-consuming effort and is not done for implanter monitoring.